Fire officials and National Grid say an underground electrical fire sparked an explosion early Saturday that sent four manhole covers flying into the air in a downtown neighborhood. It was the third such incident in less than six months.

Utility spokesman Patrick Stella tells local media outlets that road salt and snow melt can erode underground cables, leading to electrical shorts and fires. But he said such situations are rare.

Stella said National Grid will work with the city on a thorough investigation of the underground infrastructure.